Regularity of the Derivatives of Solutions to Certain Degenerate Elliptic Equations Jeff Lewis Indiana University Mathematics Journal 1983 text pdf 10.1512/iumj.1983.32.32058 10.1512/iumj.1983.32.32058 en Indiana Univ. Math. J. 32 (1983) 849 - 858 state-of-the-art mathematics http://iumj.org/access/